1. Field of the Invention
The present invention relates to a clutchless piston type variable displacement compressor, and more particularly, to a clutchless piston type variable displacement compressor which controls the inclined angle of a swash plate by utilizing the pressure differential between a crank chamber and a suction chamber to supply gas in a discharge pressure area to the crank chamber and to discharge the gas in the crank chamber to a suction pressure area, thereby adjusting the pressure in the crank chamber.
2. Description of the Related Art
In general, compressors are used in vehicles to supply compressed refrigerant gas to the vehicle's air conditioning system. To maintain air temperature inside the vehicle at a level comfortable for passengers in the vehicle, it is important to utilize a compressor having a controllable displacement. One known compressor of this type controls the inclined angle of a swash plate, tiltably supported on a rotary shaft, based on the difference between the pressure in a crank chamber and the suction pressure, and converts the rotational motion of the swash plate to the reciprocal linear motion of each piston.
In the conventional compressor, an electromagnetic clutch is provided between an external driving source, such as the vehicle's engine, and the rotary shaft of the compressor. Power transmission from the driving source to the rotary shaft is controlled by the ON/OFF action of this clutch. When power transmission from the driving source to the rotary shaft is interrupted, the compressor's displacement of refrigerant gas is set to zero. At the time when the electromagnetic clutch is activated or deactivated, the clutch's action generates a shock generally detrimental not only to the compressor but also to the overall driving comfort experienced by the vehicle's passengers. Further, the provision of the electromagnetic clutch increases the overall weight of the compressor.
To solve the above shortcoming, U.S. Pat. No. 5,173,032 issued Dec. 22, 1992 to Taguchi et al., discloses a compressor designed to set the displacement amount to zero without using an electromagnetic clutch. In such a clutchless system, the compressor runs even when no cooling is needed. With such type of compressors, it is important that when cooling is unnecessary, the discharge displacement be reduced as much as possible to prevent the evaporator from undergoing frosting. Under these conditions, it is also important to stop the circulation of the refrigerant gas through the compressor, and its external refrigerant circuit.
The compressor described in U.S. Pat. No. 5,173,032 is designed to block the flow of gas into the suction chamber in the compressor from the external refrigerant circuit by the use of an electromagnetic valve. This valve selectively allows for the circulation of the gas through the external refrigerant circuit and the compressor. When the gas circulation is blocked by the valve, the pressure in the suction chamber drops and the control valve responsive to that pressure completely opens. This complete opening of the control valve allows the gas in the discharge chamber to flow into the crank chamber, which in turn raises the pressure inside the crank chamber. The gas in the crank chamber is supplied to the suction chamber. Accordingly, a short circulation path is formed which passes through the cylinder bores, the discharge chamber, the crank chamber, the suction chamber and back to the cylinder bores.
As the pressure in the suction chamber decreases, the suction pressure in the cylinder bores falls, causing an increase in the difference between the pressure in the crank chamber and the suction pressure in the cylinder bores. This pressure differential in turn minimizes the inclination of the swash plate which reciprocates the pistons. As a result, the discharge displacement and the driving torque needed by the compressor are minimized, thus reducing power loss as much as possible when cooling is unnecessary.
The aforementioned electromagnetic valve performs a simple ON/OFF action to instantaneously stop the gas flow from the external refrigerant circuit into the suction chamber. Naturally, when the valve is off, the amount of gas supplied into the cylinder bores from the suction chamber decreases drastically. This rapid decrease in the amount of gas flowing into the cylinder bores likewise causes a rapid decrease in the discharge displacement and discharge pressure. Consequently, the driving torque needed by the compressor is drastically reduced over a short period of time.
When the electromagnetic valve switches to the ON position, the gas flow from the external refrigerant circuit to the suction chamber instantaneously starts again. Accordingly, the amount of gas supplied to the cylinder bores from the suction chamber quickly increases and the discharge displacement and discharge pressure quickly increase. Consequently, the driving torque needed by the compressor undergoes a rapid rise over a short period of time. This variation in torque caused by the ON/OFF action of the electromagnetic valve, however, prevents shock suppression which is the primary purpose of the clutchless system.